1. Field of the Invention
This invention relates to a method for the pretreatment of a semiconductor substrate in the production of ICs by ion implantation of the semiconductor substrate.
2. Description of the Prior Art
When a substrate or wafer is cut out of, for example, undoped GaAs single crystal pulled by the liquid encapsulated Czochralsky method (LEC method) and subjected to ion-implantation to prepare IC, the uniformity to FET (field-effect transistor) characteristics in the wafer surface are not good. For example, when FETs are formed by ion implantation and subjected to measurement of the source-drain electric current, there is a dispersion of about 30% across the wafer. In order to obtain a good IC, this dispersion should be less than several percent. The dispersion is said to correspond to the distribution of dislocation density of a crystal, which is possibly due to the fact that the quantity of an impurity varies with the dislocation density through the reaction of dislocation and impurity.
Generally, an impurity in a material will be precipitated when exceeding its solubility, while it will be uniformly distributed when not exceeding its solubility. If even a single crystal by LEC method is held at a high temperature for a sufficiently long time, impurities must uniformly be distributed, since the higher is the temperature, the higher is the solubility. If there is a dislocation in a crystal, however, the dislocation attracts impurities (gettering action) when the bond energy of the dislocation and impurities is equal to the thermal energy of the impurities during cooling. In said case, the thermal energy is represented by kT wherein k is the Boltzmann constant and T is the absolute temperature, and kT is larger than the energy required for diffusion of the impurities. Thus, there occurs some distribution in the concentration of impurities on the dislocation rich sites and poor sites in a crystal.
When a single crystal is grown, the solid-liquid interface is not flat as exemplified in FIG. 1. The concentration of impurities along solid-liquid interface 1 is constant, but when a wafer is prepared by slicing along line 2, there occurs a nonuniform distribution of impurities as shown in FIG. 1(b).